The present invention generally relates to thermal inkjet systems, and more particularly to an inkjet printhead having driver circuitry thereon which communicates with the printing resistors and other components of the printhead using a specialized conductive system.
A substantial demand exists for printing systems of high efficiency and resolution. To satisfy this demand, thermal inkjet cartridges have been developed which print in a rapid and efficient manner. These cartridges include an ink reservoir in fluid communication with a substrate having a plurality of resistors thereon. Selective activation of the resistors causes thermal excitation of the ink and expulsion thereof from the cartridge. Representative thermal inkjet systems are discussed in U.S. Pat. No. 4,500,895 to Buck et al., U.S. Pat. No. 4,513,298 to Scheu, U.S. Pat. No. 4,794,409 to Cowger et al., the Hewlett-Packard Journal, Vol. 36, No. 5 (May 1985), and the Hewlett-Packard Journal, Vol. 39, No. 4 (Aug. 1988).
In recent years, research has been conducted in order to increase the degree of print resolution and quality of thermal inkjet printing systems. Print resolution necessarily depends on the number of printing resistors formed on the cartridge substrate. Modern circuit fabrication techniques allow the placement of substantial quantities of resistors on a single printhead substrate. However, the number of resistors applied to the substrate is limited by the conductive components used to electrically connect the cartridge to external pulse driver circuitry in the printer unit. Specifically, an increasingly large number of resistors requires a correspondingly large number of interconnection pads, leads, and the like. This causes greater manufacturing/production costs, and increases the probability that defects will occur during the manufacturing process.
In order to solve this problem, thermal inkjet printheads have been developed which incorporate pulse driver circuitry (e.g. metal oxide semiconductor field effect (MOSFET) transistors) directly on the printhead substrate with the resistors. This development is described in U.S. Pat. No. 4,719,477 to Hess. The incorporation of driver circuitry on the printhead substrate in this manner reduces the number of interconnect components need to electrically connect the cartridge to the printer unit. This results in an improved degree of production and operating efficiency.
The integration of driver components and printing resistors onto a common substrate also results in a need for specialized, multi-layer connective circuitry so that the driver transistors can communicate with the resistors and other portions of the printing system. Typically, this connective circuitry involves a plurality of separate conductive layers, each being formed using conventional circuit fabrication techniques. However, this procedure again results in increased production costs and diminished manufacturing efficiency. The present invention involves a unique conductive system for electrically connecting the driver transistors with the transistors) integrally formed thereon. Each resistor is produced by the application of a layer of resistive material onto the substrate. The layer of resistive material preferably consists of a composition selected from the group consisting of polycrystalline silicon, a co-sputtered mixture of tantalum and aluminum, and tantalum nitride. The layer of resistive material is applied so that it is in direct physical engagement with the electrical contact regions of the drive transistors (e.g., the source, gate, and drain of MOSFET transistors). A layer of conductive material (e.g., aluminum, gold, or copper) is positioned on selected portions of the layer of resistive material in order to form covered sections of the resistive material and uncovered sections thereof. The uncovered sections ultimately function as heating resistors in the printhead. The covered sections are used to form continuous conductive links between the electrical contact regions of the transistors and other components in the printing system (e.g. the heating resistors). Thus, the layer of resistive material performs dual functions: (1) as heating resistors in the system, and (2) as direct conductive pathways to the drive transistors. This is a significant development, and substantially eliminates the need to use multiple layers for carrying out these functions.
A selected portion of protective material is then applied to the covered and uncovered sections of resistive material. Thereafter, an orifice plate member having a plurality of openings therethrough is positioned on the protective material. Beneath the opening, a section of the protective material is removed in order to from an ink-receiving cavity thereunder. Positioned below each cavity is one of the heating resistors formed as described above. The activation of each resistor by its associated driver transistor causes the resistor to printing resistors and other necessary components. The invention uses a minimal number of conductive layers which are arranged in a special manner in order to reduce the number of production steps. The resulting product operates in a highly efficient manner, and is economically manufactured compared with previous production methods.